This invention relates to a process for separating carbon dioxide from a gas mixture by using an adsorbent. More particularly, this invention relates to an improved process for separating and purifying carbon dioxide by a pressure-swing adsorption method (hereinafter referred to as "PSA method").
In a process for separating carbon dioxide by selectively adsorbing a carbon dioxide component in a gas mixture using an adsorbent containing carbon, purging the adsorbent zone with desorbed carbon dioxide and desorbing the carbon dioxide as disclosed in Published Unexamined Japanese patent application No. 59-173116, a pretreating equipment (e.g. an impurity removing equipment) is provided when impurities such as sulfur, and the like which deteriorate the adsorbent are present in the gas mixture used as raw material gases. But there usually have been no attention to moisture content, so that no water removal step is provided.
The present inventors found that in a process for adsorbing and separating carbon dioxide from a gas mixture by using the PSA method, when the gas mixture was supplied to an adsorbent in a wet gas state, the moisture was adsorbed on a part of the adsorbent to reduce the effective pore area of the adsorbent for the aimed gas component, which resulted in reducing the amounts of adsorption and desorption, lowering the recovery and lowering the purity of the product. This tendency is remarkable when the moisture content in the gas mixture, that is the dew point, becomes higher.
On the other hand, the gas mixture supplied as raw material gases is usually wet gases in a saturated state at the temperature supplied. It is a very rare case that the gas mixture is dry without pretreatment.
Further, the present inventors found that when the adsorption and desorption were conducted under ambient temperatures, there took place undesirably deviations in adsorption and desorption ability (the recovery, the purity of product, etc.) due to deviations of the ambient temperature.